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The influence of Ta2O5 additions on the thermal properties and crystallization kinetics of a lithium zinc silicate glass
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Journal of Non-Crystalline Solids 354 (2008) 301–310 www.elsevier.com/locate/jnoncrysol
The influence of Ta2O5 additions on the thermal properties and crystallization kinetics of a lithium zinc silicate glass I.W. Donald *, B.L. Metcalfe, L.A. Gerrard, S.K. Fong Materials Science Research Division, AWE, Aldermaston, Berkshire, UK Available online 7 November 2007
Abstract The lithium zinc silicate glass exhibits two crystallization exotherms corresponding to the formation of Li2ZnSiO4 and silica phases, respectively. The silica phases include tridymite and cristobalite. The influence of Ta2O5 additions on the thermal properties and nonisothermal crystallization kinetics of this glass, including nucleation rate maxima and activation energies for crystallization, has been determined by differential scanning calorimetry and X-ray diffraction. It has been found that Ta2O5 affects the crystallization behavior markedly, inhibiting the crystallization of high thermal expansion silica phases at lower concentrations, whilst at higher concentrations also suppressing crystallization of the Li2ZnSiO4 phase. At the higher concentrations, these phases are replaced by small amounts of LiTaO3 and LiTaSiO5. Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 65.60.+a; 64.70.Pf; 61.10.Nz Keywords: Crystallization; Glass-ceramics; Nucleation; Diffraction and scattering measurements; X-ray diffraction; Oxide glasses; Alkali silicates; Thermal properties; Calorimetry; X-rays
1. Introduction Glass-ceramics from the lithium zinc silicate, LZS, system are a versatile class of material, first reported by McMillan and co-workers in the 1960s [1], and which in the past have been employed in glass-ceramic-to-metal seal applications [2]. Quite recently, renewed interest has been shown in LZS glass-ceramics for sealing applications [3–6]. In this study, the influence on the crystallization behavior of Ta2O5 additions to a lithium zinc silicate glass, used for sealing to Ni-based superalloys and stainless steel [7–9], has been examined. The unmodified lithium zinc silicate glass exhibits two well-defined crystallization exotherms corresponding to the formation of Li2ZnSiO4 and silica phases [10,11]. The silica phases may include tridymite, cristobalite and quartz, which *
Corresponding author. E-mail address: [email protected] (I.W. Donald).
can give rise to relatively high thermal expansion glassceramics. An early study [11] of the effect of a variety of Groups IV, V and VI transition metal oxide (TMO) additions on this glass indicated that some additions, for example, Nb2O5 and Ta2O5, may decrease crystallization activation energies and increase the proportion of lower expansion phases in the crystallized product. It can therefore be assumed that incorporation of these oxides in varying concentrations may lead to practical glassceramics offering a wide range of thermal expansion coefficients. In the present contribution the influence of Ta2O5 in varying concentrations, on the properties of LZS glass is described in some detail. Data presented include glass transition and crystallization temperatures, activation energies for crystallization and structural relaxation, and details of the crystallization products obtained. Thermal expansion behavior will be reported in a subsequent paper.
0022-3093/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.06.102
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2. Experimental A lithium zinc silicate, LZS, glass of analyzed composition 17.84 Li2O–5.25 Na2O–17.73 ZnO–4.31 B2O3–1.23 P2O5–53.64 SiO2 (mol%), was used in this study. This was taken from a 200 kg batch of glass prepared using a Joule melter. To this glass various amounts of Ta2O5 over the range 0.2–8 mol% were added to LZS frit and the mixture melted in a Pt–5% Rh crucible for 1 h at 1450 °C. The melt was subsequently poured into deionized water and the resulting frit remelted to yield homogeneous products. The thermal characteristics of as-quenched glasses of standard particle size 600–1000 lm were monitored using a Netzsch heat-flux DSC Model 404, capable of measuring to 0.1 K, at a standard heating rate of 10 K/min. A minimum of five samples was employed for each determination. The properties measured included the glass transition temperature, Tg, peak crystallization temperatures, Tx1, Tx2, start of crystallization, Tx1(s), crystallization enthalpy, DH, and the temperature corresponding to the end of the melting range, Tliq. Samples of mass 30 ± 1 mg were contained in alumina pans for thermal property measurements and in Pt pans for kinetic analyses. The usual precautions were taken to ensure the accuracy of all data gathered [12,13]. These included temperature calibration employing standard reference materials with well established temperature transitions, as reported elsewhere [12,14], at various heating rates over the temperature ranges employed in the activation energy studies, in addition to enthalpy calibration, also using standard reference materials. In order to minimize the influence of temperature gradients within samples it was ensured that glass particles were evenly distributed as a single layer in the sample pans. An assessment of whether bulk or surface crystallization was the predominant crystallization mechanism for each glass was carried out using the method of Thakur and Thiagarajan [15] in which the crystallization temperature is monitored as a function of glass particle size. In the present study, a fine particle size of 6212 lm was used and the results compared with the standard 600–1000 lm samples. If surface crystallization is the predominant mechanism a strong dependence on particle size would be expected, with DT = Tx(fine) Tx(coarse) giving high negative values. An assessment of the thermal stability of each glass was also made by measuring the working range of the glass, which is the temperature interval between Tg and the start of crystallization, DTTS = Tx1(s) Tg. The smaller this temperature interval, the lower the thermal stability of the glass, and the more difficult it is to avoid partial crystallization during any working operation, including sintering. Temperatures corresponding to maximum nucleation rates (hereafter referred to as ‘optimum’ nucleation temperatures) for selected glass composition were determined using the method of Marotta et al. [16]. A standard glass particle size of 600–1000 lm was used and samples were nucleated in situ in the DSC for 1 h at temperatures in the range 430–500 °C. Peak crystallization temperatures
were subsequently monitored by heating the samples through the crystallization regime at 10 K/min. The optimum nucleation temperatures were then determined from plots of DTnuc = (Tp(as-quenched)) Tp(nucleated)) against the temperature of nucleation, Tnuc. In all cases, clear maxima were noted. Apparent activation energies for crystallization, Ec, were determined from the variation in peak crystallization temperature with heating rate employing the modified Kissinger method as described by Matusita and Saaka [17]: lnðbn =T 2p Þ ¼ ðmEc =RT p Þ þ constant;
ð1Þ
where b is the heating rate, Tp is the peak crystallization temperature, and n and m are constants which depend on the crystallization mechanism. It is very important in measurements of this nature that the heating rates employed are not as high as to cause unnecessary temperature gradients within the sample. In this study, a minimum of five different heating rates in the range 2–20 K/min were employed for these determinations (normally 2, 5, 10, 15 and 20 K/min, although intermediate heating rates were additionally employed in some instances). Peak temperatures were corrected for heating rate to an accuracy of 0.1 K. Although the precise crystallization mechanism must be known in order to ascribe appropriate values to n and m, in the case of a given series of glasses where the crystallization behavior and the crystallization products obtained are likely to be similar, and where a trend in behavior is sought, it may be justifiable to employ the unmodified Kissinger equation with n = m = 1 [18]. It must be emphasized, however, that without prior knowledge of the precise crystallization mechanism and the morphology of the crystalline phases formed, values for E obtained using non-isothermal methods must be treated with extreme caution and used for qualitative comparative purposes only [12,13,18]. Alternatively, it has been proposed [19] that a more reliable estimate for n may be obtained from the slope of a plot of lnð lnð1 xÞÞ versus ln b;
ð2Þ
where x is the fraction crystallized at a given temperature. A value for m can then be found using the relationship n = m + 1. It may then be more appropriate to employ these values for n and m in the modified Kissinger relationship. The activation energy for structural relaxation around the glass transition was also determined for some samples employing the method described by Moynihan [20]. In this technique, the glass is cooled through the glass transition region at a given cooling rate and then heated back through the transition at the same rate. This is repeated for different cooling/heating rate cycles and the activation energy for structural relaxation, Erelax, calculated from the relationship: lnðbÞ ¼ Erelax =RT g :
ð3Þ
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Tg/Tliq. This parameter is related to the critical cooling rate for glass formation for glasses of similar composition and melt viscosity [21], with higher values corresponding to lower critical cooling rates and therefore enhanced GFA. XRD analysis was performed on selected samples employing a Bruker D8 Advance powder diffractometer
Intensity
Cooling/heating cycles in the range 5–25 K/min were used in these determinations, with a minimum of five different cooling/heating cycles employed (generally 5, 10, 15, 20 and 25 K/min). An estimate of the glass-forming ability, GFA, of each glass was made utilizing the reduced glass temperature,
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0.5 mol.% 1.5 mol.% 2.0 mol.% 4.0 mol.% 5.0 mol.% 6.5 mol.% 8.0 mol.%
5
10
2
30
40
50
Two-Theta Fig. 1. Representative XRD patterns of glasses. Note that all concentrations are X-ray amorphous; also note the shoulder on the amorphous peak at the higher Ta2O5 concentrations.
Fig. 2. Representative DSC traces for Ta2O5 loaded glasses: (a) DSC traces for 0.2 and 0.5 mol% Ta2O5 loaded glasses and (b) DSC traces for 2–8 mol% Ta2O5 loaded glasses.
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with Bragg–Brentano flat plane geometry. The 2h scans were made between 5° and 60°, step width of 0.02° and a time per step of 4 s, with Cu Ka1 radiation
˚ ). Preparation of the samples used a sim(k = 1.54056 A ple top pack loading method for an acquired smooth surface.
Table 1 Thermal properties of the LZS glass containing Ta2O5 additions TMO
Tg(s) (°C)
Tg(mp) (°C)
Tx1(s) (°C)
DTTS (K)
Tx1 (°C)
Tx2 (°C)
Tliq (°C)
Tg(s)/Tliq (K/K)
None Ta(0.2) Ta(0.5) Ta(1) Ta(1.5) Ta(2) Ta(3) Ta(4) Ta(5) Ta(6.5) Ta(8)
442 ± 1 444 ± 2 447 ± 2 447 ± 3 452 ± 2 457 ± 1 459 ± 1 468 ± 3 475 ± 2 488 ± 5 498 ± 2
452 ± 1 455 ± 1 457 ± 1 458 ± 0 464 ± 1 467 ± 0 471 ± 0 481 ± 1 491 ± 1 505 ± 0 515 ± 2
621 ± 1 626 ± 1 636 ± 1 642 ± 1 666 ± 1 678 ± 1 691 ± 1 729 ± 3 722 ± 9 Not resolved Not resolved
169 171 179 184 202 211 220 248 231 – –
655 ± 1 660 ± 2 670 ± 1 678 ± 0 700 ± 1 713 ± 1 729 ± 1 772 ± 2 797 ± 5 783 ± 6 821 ± 26
720 ± 1 732 ± 0 732 ± 0 738 ± 1 756 ± 1 772 ± 2 No peak No peak No peak No peak No peak
951 ± 1 944 ± 1 944 ± 1 940 ± 0 940 ± 1 938 ± 0 938 ± 1 932 ± 1 933 ± 2 933 ± 7 985 ± 15
0.584 0.598 0.600 0.603 0.608 0.611 0.614 0.626 0.633 0.645 0.626
Tg(s) is the glass transition start temperature; Tg(mp) is the corresponding mid-point temperature; Tx(s) is the extrapolated peak start of the first or only crystallization exotherm; Tx1,2 are the peak crystallization temperatures; Tliq is the end of the melting range. DTTS is the working range defined as Tx1(s)– Tg(mp), and Tg(mp)/Tliq is the reduced glass temperature. Quoted uncertainties are the standard deviations.
Fig. 3. DSC traces for as-quenched and nucleated LZS glass containing Ta2O5. (a) DSC traces for as-quenched and nucleated LZS glass containing 0.5 mol% Ta2O5. Glass nucleated for 1 h at 465 °C. (b) Traces for as-quenched and nucleated LZS glass containing 3 mol% Ta2O5. Glass nucleated for 1 h at 475 °C.
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3. Results 30
peak 1
20 10
peak 2
0 420
440
460
480
500
520
Nucleation temperature (°C) 60
Tnuc (K)
50 peak 1
40 30 20
peak 2
10 0 420
440
460
480
500
520
Nucleation temperature (°C) 60 50 peak 1
Tnuc (K)
40 30 20 No second peak
10 0 420
440
460
480
500
520
Nucleation temperature (°C) Fig. 4. Plots of variation in peak crystallization temperature between asquenched and nucleated LZS glasses as a function of the nucleating temperature. (a) Plot of variation in peak crystallization temperature between as-quenched and nucleated unmodified LZS glass as a function of the nucleating temperature. (b) Plot of variation in peak crystallization temperature between as-quenched and nucleated LZS glass doped with 2 mol% Ta2O5 as a function of the nucleating temperature. (c) Plot of variation in peak crystallization temperature between as-quenched and nucleated LZS glass doped with 3 mol% Ta2O5 as a function of the nucleating temperature.
500 490
Tnuc (K)
All the glass compositions examined were noted to be fully amorphous as determined by XRD, even at the 8 mol% Ta2O5 level. Representative patterns are shown in Fig. 1. A shoulder can be observed on the high angle side of the amorphous peak as the concentration of Ta2O5 increases. This may indicate a degree of structural ordering in these higher Ta2O5 glasses. Representative DSC traces of a selection of the glasses are shown in Fig. 2 and the thermal properties are summarized in Table 1. The strong influence of Ta additions on the glass transition temperature of the LZS glass is also summarized in Table 1, Tg(s) increasing from around 442 °C to 498 °C for an 8 mol% addition. The DSC traces shown in Fig. 2 illustrate the increasing stabilizing influence of Ta additions on crystallization, where it may be noted that crystallization is virtually eliminated for concentrations >5 mol%. The heat of crystallization, DH, of the first crystallization peak for the LZS glass is 213 ± 3 J/g and for the second 40 ± 2 J/g, whilst a value of around 4 J/g is noted for the small single peak observed with the 8% Ta sample. As summarized in Table 1 the working range of the glasses as measured by DTTS is improved by the Ta additions. The GFA of the glasses, as defined by the reduced glass temperature, is marginally enhanced by all additions. In general, the nucleating efficiency of the as-quenched glasses is very good, with DT values between coarse and fine glass samples generally <10 K, this suggesting predominantly bulk crystallization. The influence of specific nucleation treatments on the glasses is nevertheless also very marked, as illustrated by the DSC traces shown in Fig. 3, and in Fig. 4 which shows Marotta plots of DTnuc against the temperature of nucleation, Tnuc. Very clear maxima are noted in these plots from which the optimum nucleation temperature for each composition can be derived. A substantial increase in optimum nucleation temperature with Ta2O5 loading is observed from around 465 °C to 490 °C, as shown in Fig. 5. Activation energies for crystallization are given in Table 2 and representative activation energy plots are shown in Fig. 6. The crystallization activation energies for asquenched glasses are reduced by all additions. It is particularly noteworthy that addition of Ta appears to suppress the formation of the high expansion silica phases. The activation energy for structural relaxation around the glass transition for the unmodified LZS glass was determined as 568 ± 15 kJ/mol whilst the value for samples loaded with 5% Ta2O5 was found to be significantly higher at 654 ± 25 kJ/mol. Structural relaxation activation energy plots comparing LZS glass with glass containing 5 mol% Ta2O5 addition are shown in Fig. 7. Summarized in Table 3 are the crystalline phases that form in samples heat-treated under standard conditions of 5 min at 950 °C to simulate a sealing cycle, followed by nucleation for 60 min at 465 °C and crystallization for 60 min at either 700 °C or 800 °C. The major crystalline
Tnuc (K)
40
480 470 460 450
0
1
2
3
4
5
6
Ta2O5 (mol%) Fig. 5. Optimum nucleation temperature as a function of Ta2O5 doping.
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Table 2 Crystallization activation energies for glasses containing TMO additions TMO
E1 (kJ/mol)
E2 (kJ/mol)
None Ta(0.2) Ta(0.5) Ta(1) Ta(1.5) Ta(2) Ta(4) Ta(5)
237 ± 2 159 ± 7 152 ± 5 157 ± 4 160 ± 3 158 ± 5 155 ± 4 200 ± 13
304 ± 2 275 ± 5 286 ± 12 276 ± 19 240 ± 12 164 ± 17a No peak No peak
a Only resolved at low heating rates 65 K/min. Quoted uncertainties are the standard deviations.
-10.5
5 4
2 1.5
1 0.5 0.2 mol% Ta2O5
ln (β/Tp2)
-11
phases include the silica polymorphs tridymite (PDF No. 83-1339) and cristobalite (PDF No. 39-1425), together with a Li2ZnSiO4 phase (PDF No. 15-0056), most probably a solid solution, and some residual glassy phases, as depicted by the XRD patterns shown in Fig. 8. There is considerably more tridymite present in the unmodified LZS crystallized at 800 °C. At the higher concentrations of Ta2O5, P2 mol%, tridymite is absent in samples crystallized at either 700 °C or 800 °C, and the proportion of cristobalite is reduced; whilst at higher concentrations still, the Li2ZnSiO4 phase is also substantially reduced and LiTaO3 (PDF No. 29-0830) and LiTaSiO5 (PDF No. 45-0644) phases appear. For 8 mol% Ta2O5 addition, samples heat treated at 700 °C are still XRD amorphous. Representative micrographs are shown in Fig. 9, which illustrate the pronounced effect Ta2O5 has on the crystallization behavior, including phase distribution and morphology. 4. Discussion
-11.5 -12 -12.5 -13 -13.5 0.9
0.95
1
1.05
1.1
1.15
1 / T (10-3K-1) Fig. 6. Representative activation energy plots for glasses containing Ta2O5 in the range 0.2–5 mol%.
3.5
ln(β)
LZS
5mol% Ta2O5
3 2.5 2 1.5 1.28
1.3
1.32
1.34
1.36
1.38
1.4
1 / Tg (10-3K-1) Fig. 7. Structural relaxation activation energy plots for LZS glass and glass doped with 5 mol% Ta2O5.
There have been very few reports detailing the effects of TMO additions, and in particular Ta2O5, on the properties of oxide glasses, and these have usually concentrated on a very small number of individual metals. For example, in the case of silicate glasses, the influence of Ti, Co, Fe, Mn, Ni and Co on the thermal conductivity of a barium borosilicate glass, where it was shown that the conductivity increases with increasing atomic weight of the metallic species [22]; the addition of Cr to a lithium silicate composition in order to increase solar absorbance [23]; and the effect of various TMO additions at the 2 mol% level on the properties of a LZS glass [11]. Some studies have also been reported for non-silicate glasses. These include phosphates where a limited number of TMO additions have been noted to influence the structure of the glasses [24– 26]; borate glasses where the addition of Fe, Co and Ni was noted to influence the electrical conductivity [27]; vanadate glasses where TMO additions were noted to enhance diffusion [28]; and tellurite glasses where TMO additions influenced the structure [29]. Differential scanning calorimeter traces of the lithium zinc silicate glass employed in the current investigation exhibit two very well-defined exotherms. It is known from previous studies [11] that the first exotherm corresponds to
Table 3 Crystalline phases formed in heat-treated glasses TMO
Crystalline phases 700 °C
Crystalline phases 800 °C
None Ta(0.2) Ta(0.5) Ta(1.0) Ta(1.5) Ta(2.0) Ta(4.0) Ta(6.5) Ta(8.0)
Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + Li2ZnSiO4 + LiTaO3 Amorphous
Tridymite + Cristobalite + Li2ZnSiO4 Tridymite + Cristobalite + Li2ZnSiO4 Tridymite + Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + LiTaSiO5 + LiTaO3
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Lithium Zinc Silicate, Li2 ZnSiO4 Cristobalite, SiO2 Tridymite, SiO2
Intensity
800°C
700°C
15
20
30
Two Theta Lithium Zinc Silicate, Li2 ZnSiO4 Cristobalite, SiO2 Tridymite, SiO
Intensity
800°C
700°C
15
20
30
Two Theta Fig. 8. Representative XRD patterns of crystallized glasses (slight shifts from their file values are observed for some of the peaks and are most likely the result of solid solution influences). (a) Unmodified LZS glass crystallized at 700 °C and 800 °C. Note the absence of tridymite peaks at 700 °C but the strong peaks at 800 °C. (b) LZS glass with addition of 2 mol% Ta2O5 crystallized at 700 °C and 800 °C. Note that tridymite is absent at both temperatures. (c) All LZS glasses from 0 to 8 mol% Ta2O5 crystallized at 700 °C. Top pattern is for the unmodified LZS; this is followed by 0.2, 0.5, 1, 2, 4, 6.5 and 8 mol% Ta2O5 modified glasses. Note the absence of tridymite peaks and the decreasing peaks from LZS and cristobalite with increasing Ta2O5, and the small proportion of LiTaO3 at the higher concentrations (4% and 6.5%); also note that the 8% composition is still X-ray amorphous at this crystallization temperature. (d) All LZS glasses crystallized at 800 °C. Top pattern is for the unmodified LZS; this is followed by 0.2, 0.5, 1, 2, 4, 6.5 and 8 mol% Ta2O5 modified glasses. Note the decreasing peaks from tridymite, cristobalite and LZS and with increasing Ta2O5 content; and also note the increasing proportions of LiTaSiO5 and LiTaO3 crystalline phases.
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Lithium Zinc Silicate, Li2ZnSiO4 Cristobalite, SiO2
Intensity
Lithium Tantalum Oxide, LiTaO3
LZS 0.2% 0.5% 1.0% 2.0% 4.0% 6.5% 8.0 %
20
25
30
35
40
Two Theta
Lithium Zinc Silicate, Li2SiO4 Cristobalite, SiO2 Tridymite, SiO2 Lithium Tantalum Oxide, LiTaO3 Lithium Tantalum Silicate, LiTaSiO5
Intensity
LZS 0.2% 0.5% 1.0% 2.0% 4.0% 6.5% 8.0 %
20
25
30
35
40
Two Theta Fig. 8 (continued)
the formation of a Li2ZnSiO4 phase, whilst the second higher temperature exotherm yields silica phases, i.e., cristobalite, quartz or tridymite, or a combination of these. It is clear that the addition of Ta2O5 to this glass can have a very pronounced influence on the thermal properties and crystallization behavior, as is evident from Tables 1–3 and Figs. 2–5, 8 and 9. Most noteworthy is the ability of Ta to suppress crystallization of the higher expansion silica phases, whilst at higher concentrations crystallization of Li2ZnSiO4 is also almost entirely eliminated and a small proportion of LiTaO3 and LiTaSiO5 phases are formed. The precise reason why Ta should have such a pronounced influence on the stability of the glass to crystallization is not clear at this time, but may be related to the ability of
this refractory TMO to enter into the structure in a network former or intermediate role, this being borne out by the marked increase in the glass transition temperature with Ta additions, noted in Table 1. Activation energies for crystallization are reduced by all Ta2O5 additions, as summarized in Table 2. It should be noted that the values shown in this table are the Kissinger values, generally applicable for surface crystallization, and therefore do not take into account the crystallization mechanism. This is considered valid for a qualitative comparison between the unmodified LZS and the various Ta2O5 additions, as the overall crystallization mechanism is likely to be similar for all these compositions. It is noteworthy that use of the Matusita and Saaka modified equation
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using values for n = m = 3, i.e. bulk crystallization, yields values for apparent activation energy approximately only 3–5% higher than the unmodified Kissinger case, and this is normally well within the experimental error associated with such values [18]. Isothermal studies on this LZS glass in the as-quenched state [13] indicate that n corresponding to the first crystallization exotherm is in the range 1.6–2.1
309
over the temperature range 635–660 °C. In the present study, using Eq. (2) a value for n in the range 1.7–2.4 over the same temperature range is determined, which is in very good agreement with the isothermal method. The major nucleating agent in this system is P2O5 and it is known [11] that, on nucleation, glass-in-glass phase separation occurs on a very fine nanometer scale. The precise effect of Ta2O5 additions on this behavior is not yet clear, but nucleation performance is undoubtedly enhanced at low concentrations, as is evident by the increase in the values for DT noted in the Marotta plots shown in Fig. 4, but is suppressed at higher concentrations, >5 mol%, when crystallization is very much less pronounced. The current work has indicated that the precise silica phases formed and their relative proportions are very dependent on the processing conditions, and particularly on the crystallization temperature employed and the amount of Ta2O5 present. As noted in Table 3 and illustrated by Fig. 9 which shows SEM micrographs of crystallized unmodified LZS and crystallized 3 and 4 mol% Ta2O5 samples, addition of Ta2O5 does indeed have a very pronounced influence on the crystallization behavior including the phase distribution and morphology. The findings of this work may have important implications for the development of glass-ceramics with tailored thermal expansion characteristics. By modifying the proportions of the intermediate and high expansion phases it may be possible to produce glass-ceramics in the lithium zinc silicate system, and in ostensibly the same LZS composition, with a wide variety of characteristics to match many different materials, both metals and ceramics. The influence of composition on thermal expansion behavior will be reported in a subsequent publication. 5. Conclusions
Fig. 9. SEM micrographs of glasses crystallized at 800 °C. (a) Unmodified LZS crystallized glass (phases present include lithium zinc silicate together with cristobalite/tridymite and residual glass). (b) Crystallized LZS glass with 3 mol% Ta2O5 addition (phases present include lithium zinc silicate, cristobalite/tridymite, lithium tantalum oxide and residual glass). (c) LZS with 4 mol% Ta2O5 addition (phases present as (b) together with lithium tantalum silicate).
(1) It is confirmed that the lithium zinc silicate glass exhibits two crystallization exotherms which correspond to the formation of lithium zinc silicate and silica phases, respectively. (2) Addition of Ta2O5 suppresses the formation of high thermal expansion silica phases, whilst at higher concentrations, it also inhibits crystallization of the Li2ZnSiO4 phase. These phases are replaced by small amounts of LiTaO3 and LiTaSiO4. (3) Thermal properties, including the glass transition temperature, glass-forming ability, crystallization temperature and optimum nucleation temperature corresponding to the maximum nucleation rate, are all increased by addition of Ta2O5. (4) Nucleation of glasses at temperatures around the glass transition leads to very significant reductions in the crystallization temperature, as noted by DSC. (5) Activation energies for crystallization are significantly reduced by all Ta2O5 additions, whilst activation energy for structural relaxation is increased.
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Acknowledgements British Crown Copyright 2006/MOD. Published with the permission of the Controller of Her Britannic Majesty’s Stationery Office. References [1] P.W. McMillan, B.P. Hodgson, Engineering 196 (1963) 366. [2] I.W. Donald, Glass-to-Metal Seals Society of Glass Technology (Sheffield), 2007. [3] B.I. Sharma, M. Goswami, P. Sengupta, V.K. Shrikhande, G.B. Kale, G.P. Kothiyal, Mater. Lett. 58 (2004) 2423. [4] I.W. Donald, B.L. Metcalfe, S.K. Fong, in: Proc. XX Int. Congr. Glass, Kyoto, Japan, 2004. [5] M. Goswami, P. Sengupta, K. Sharma, R. Kumar, V.K. Shrikhande, J.M.F. Ferreira, G.P. Kothiyal, Ceramics International 33 (2007) 863. [6] I.W. Donald, B.L. Metcalfe, L.A. Gerrard, S.K. Fong, Advances in Science and Technology, vol. 45, Trans Tech Publications, Switzerland, 2006, p. 1520. [7] B.L. Metcalfe, I.W. Donald, New Materials and their Applications, in: D. Holland (Ed.), Inst. Physics Conf. Series No. 111, IOP, Bristol, 1990, p. 469. [8] B.L. Metcalfe, I.W. Donald, D.J. Bradley, in: R. Morrell, G. Partridge, (Eds.), Brit. Ceram. Proc. No. 48, Surfaces and interfaces, Brit. Ceram. Soc. (Stoke-on-Trent), 1991, p. 177. [9] I.W. Donald, B.L. Metcalfe, D.J. Bradley, S.E. Battersby, Ceramics: Charting the Future, in: P. Vincenzini (Ed.), Proc. 8th. CIMTEC World Ceramics Congress, Techna Srl., 1995, p. 2239. [10] I.W. Donald, B.L. Metcalfe, D.J. Wood, J.R. Copley, J. Mater. Sci. 8 (1989) 3892.
[11] I.W. Donald, B.L. Metcalfe, E.A.P. Morris, J. Mater. Sci. 27 (1992) 2979. [12] I.W. Donald, J. Mater. Sci. 30 (1995) 904. [13] I.W. Donald, J. Non-Cryst. Solids 345&346 (2004) 120. [14] T. Hatakeyama, Z. Lui (Eds.), Handbook of Thermal Analysis, John Wiley, Chichester, 1998. [15] R.L. Thakur, S. Thiagarajan, Glass Ceram. Bull. 13 (1966) 33. [16] A. Marotta, A. Buri, F. Branda, J. Mater. Sci. 16 (1981) 341. [17] K. Matusita, S. Saaka, Bull. Inst. Chem. Res. Kyoto University 59 (1981) 159. [18] I.W. Donald, B.L. Metcalfe, S.K. Fong, L.A. Gerrard, J. Non-Cryst. Solids 352 (2006) 2993. [19] H. Yinnon, D.R. Uhlmann, J. Non-Cryst. Solids 54 (1983) 253. [20] C.T. Moynihan, J. Amer. Ceram. Soc. 76 (1993) 1081. [21] H.A. Davies, Phys. Chem. Glasses 17 (1976) 159. [22] N.A. Ghoneim, A.A. Ahmed, S. Gharib, Thermochim. Acta 71 (1983) 43. [23] G. Fernandez-Arroyo, C. Moure, J.R. Jurado, J. Non-Cryst. Solids 80 (1986) 533. [24] P. Bergo, W.M. Pontuschka, J.M. Prison, C.C. Motta, J.R. Martinelli, J. Non-Cryst. Solids 348 (2004) 84. [25] O. Cozar, I. Ardelean, V. Simon, V. Mih, N. Vedean, J. Magn. Magn. Mater. 196&197 (1999) 269. [26] N. Zotov, A. Kirfel, B. Beuneu, R. Delaplane, D. Hohlwein, F. Reinauer, R. Glaum, Physics B: Condens. Matter 350 (2004) E1071. [27] A.A. Soliman, S.A. Aly, H. Frhan, Y.M. Abo-Zeid, Radiat. Phys. Chem. 54 (1999) 499. [28] N. Machida, R. Fuchida, T. Minami, Solid State Ionics 35 (1989) 295. [29] T.A. Mohamed, I. Shaltout, K.M. Al Yahyaei, Spectrochim. Acta A: Molec. Biomolec. Spectros. 64 (2006) 106.
Journal of Non-Crystalline Solids 354 (2008) 301–310 www.elsevier.com/locate/jnoncrysol
The influence of Ta2O5 additions on the thermal properties and crystallization kinetics of a lithium zinc silicate glass I.W. Donald *, B.L. Metcalfe, L.A. Gerrard, S.K. Fong Materials Science Research Division, AWE, Aldermaston, Berkshire, UK Available online 7 November 2007
Abstract The lithium zinc silicate glass exhibits two crystallization exotherms corresponding to the formation of Li2ZnSiO4 and silica phases, respectively. The silica phases include tridymite and cristobalite. The influence of Ta2O5 additions on the thermal properties and nonisothermal crystallization kinetics of this glass, including nucleation rate maxima and activation energies for crystallization, has been determined by differential scanning calorimetry and X-ray diffraction. It has been found that Ta2O5 affects the crystallization behavior markedly, inhibiting the crystallization of high thermal expansion silica phases at lower concentrations, whilst at higher concentrations also suppressing crystallization of the Li2ZnSiO4 phase. At the higher concentrations, these phases are replaced by small amounts of LiTaO3 and LiTaSiO5. Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. PACS: 61.43.Fs; 65.60.+a; 64.70.Pf; 61.10.Nz Keywords: Crystallization; Glass-ceramics; Nucleation; Diffraction and scattering measurements; X-ray diffraction; Oxide glasses; Alkali silicates; Thermal properties; Calorimetry; X-rays
1. Introduction Glass-ceramics from the lithium zinc silicate, LZS, system are a versatile class of material, first reported by McMillan and co-workers in the 1960s [1], and which in the past have been employed in glass-ceramic-to-metal seal applications [2]. Quite recently, renewed interest has been shown in LZS glass-ceramics for sealing applications [3–6]. In this study, the influence on the crystallization behavior of Ta2O5 additions to a lithium zinc silicate glass, used for sealing to Ni-based superalloys and stainless steel [7–9], has been examined. The unmodified lithium zinc silicate glass exhibits two well-defined crystallization exotherms corresponding to the formation of Li2ZnSiO4 and silica phases [10,11]. The silica phases may include tridymite, cristobalite and quartz, which *
Corresponding author. E-mail address: [email protected] (I.W. Donald).
can give rise to relatively high thermal expansion glassceramics. An early study [11] of the effect of a variety of Groups IV, V and VI transition metal oxide (TMO) additions on this glass indicated that some additions, for example, Nb2O5 and Ta2O5, may decrease crystallization activation energies and increase the proportion of lower expansion phases in the crystallized product. It can therefore be assumed that incorporation of these oxides in varying concentrations may lead to practical glassceramics offering a wide range of thermal expansion coefficients. In the present contribution the influence of Ta2O5 in varying concentrations, on the properties of LZS glass is described in some detail. Data presented include glass transition and crystallization temperatures, activation energies for crystallization and structural relaxation, and details of the crystallization products obtained. Thermal expansion behavior will be reported in a subsequent paper.
0022-3093/$ - see front matter Crown Copyright Ó 2007 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.06.102
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2. Experimental A lithium zinc silicate, LZS, glass of analyzed composition 17.84 Li2O–5.25 Na2O–17.73 ZnO–4.31 B2O3–1.23 P2O5–53.64 SiO2 (mol%), was used in this study. This was taken from a 200 kg batch of glass prepared using a Joule melter. To this glass various amounts of Ta2O5 over the range 0.2–8 mol% were added to LZS frit and the mixture melted in a Pt–5% Rh crucible for 1 h at 1450 °C. The melt was subsequently poured into deionized water and the resulting frit remelted to yield homogeneous products. The thermal characteristics of as-quenched glasses of standard particle size 600–1000 lm were monitored using a Netzsch heat-flux DSC Model 404, capable of measuring to 0.1 K, at a standard heating rate of 10 K/min. A minimum of five samples was employed for each determination. The properties measured included the glass transition temperature, Tg, peak crystallization temperatures, Tx1, Tx2, start of crystallization, Tx1(s), crystallization enthalpy, DH, and the temperature corresponding to the end of the melting range, Tliq. Samples of mass 30 ± 1 mg were contained in alumina pans for thermal property measurements and in Pt pans for kinetic analyses. The usual precautions were taken to ensure the accuracy of all data gathered [12,13]. These included temperature calibration employing standard reference materials with well established temperature transitions, as reported elsewhere [12,14], at various heating rates over the temperature ranges employed in the activation energy studies, in addition to enthalpy calibration, also using standard reference materials. In order to minimize the influence of temperature gradients within samples it was ensured that glass particles were evenly distributed as a single layer in the sample pans. An assessment of whether bulk or surface crystallization was the predominant crystallization mechanism for each glass was carried out using the method of Thakur and Thiagarajan [15] in which the crystallization temperature is monitored as a function of glass particle size. In the present study, a fine particle size of 6212 lm was used and the results compared with the standard 600–1000 lm samples. If surface crystallization is the predominant mechanism a strong dependence on particle size would be expected, with DT = Tx(fine) Tx(coarse) giving high negative values. An assessment of the thermal stability of each glass was also made by measuring the working range of the glass, which is the temperature interval between Tg and the start of crystallization, DTTS = Tx1(s) Tg. The smaller this temperature interval, the lower the thermal stability of the glass, and the more difficult it is to avoid partial crystallization during any working operation, including sintering. Temperatures corresponding to maximum nucleation rates (hereafter referred to as ‘optimum’ nucleation temperatures) for selected glass composition were determined using the method of Marotta et al. [16]. A standard glass particle size of 600–1000 lm was used and samples were nucleated in situ in the DSC for 1 h at temperatures in the range 430–500 °C. Peak crystallization temperatures
were subsequently monitored by heating the samples through the crystallization regime at 10 K/min. The optimum nucleation temperatures were then determined from plots of DTnuc = (Tp(as-quenched)) Tp(nucleated)) against the temperature of nucleation, Tnuc. In all cases, clear maxima were noted. Apparent activation energies for crystallization, Ec, were determined from the variation in peak crystallization temperature with heating rate employing the modified Kissinger method as described by Matusita and Saaka [17]: lnðbn =T 2p Þ ¼ ðmEc =RT p Þ þ constant;
ð1Þ
where b is the heating rate, Tp is the peak crystallization temperature, and n and m are constants which depend on the crystallization mechanism. It is very important in measurements of this nature that the heating rates employed are not as high as to cause unnecessary temperature gradients within the sample. In this study, a minimum of five different heating rates in the range 2–20 K/min were employed for these determinations (normally 2, 5, 10, 15 and 20 K/min, although intermediate heating rates were additionally employed in some instances). Peak temperatures were corrected for heating rate to an accuracy of 0.1 K. Although the precise crystallization mechanism must be known in order to ascribe appropriate values to n and m, in the case of a given series of glasses where the crystallization behavior and the crystallization products obtained are likely to be similar, and where a trend in behavior is sought, it may be justifiable to employ the unmodified Kissinger equation with n = m = 1 [18]. It must be emphasized, however, that without prior knowledge of the precise crystallization mechanism and the morphology of the crystalline phases formed, values for E obtained using non-isothermal methods must be treated with extreme caution and used for qualitative comparative purposes only [12,13,18]. Alternatively, it has been proposed [19] that a more reliable estimate for n may be obtained from the slope of a plot of lnð lnð1 xÞÞ versus ln b;
ð2Þ
where x is the fraction crystallized at a given temperature. A value for m can then be found using the relationship n = m + 1. It may then be more appropriate to employ these values for n and m in the modified Kissinger relationship. The activation energy for structural relaxation around the glass transition was also determined for some samples employing the method described by Moynihan [20]. In this technique, the glass is cooled through the glass transition region at a given cooling rate and then heated back through the transition at the same rate. This is repeated for different cooling/heating rate cycles and the activation energy for structural relaxation, Erelax, calculated from the relationship: lnðbÞ ¼ Erelax =RT g :
ð3Þ
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Tg/Tliq. This parameter is related to the critical cooling rate for glass formation for glasses of similar composition and melt viscosity [21], with higher values corresponding to lower critical cooling rates and therefore enhanced GFA. XRD analysis was performed on selected samples employing a Bruker D8 Advance powder diffractometer
Intensity
Cooling/heating cycles in the range 5–25 K/min were used in these determinations, with a minimum of five different cooling/heating cycles employed (generally 5, 10, 15, 20 and 25 K/min). An estimate of the glass-forming ability, GFA, of each glass was made utilizing the reduced glass temperature,
303
0.5 mol.% 1.5 mol.% 2.0 mol.% 4.0 mol.% 5.0 mol.% 6.5 mol.% 8.0 mol.%
5
10
2
30
40
50
Two-Theta Fig. 1. Representative XRD patterns of glasses. Note that all concentrations are X-ray amorphous; also note the shoulder on the amorphous peak at the higher Ta2O5 concentrations.
Fig. 2. Representative DSC traces for Ta2O5 loaded glasses: (a) DSC traces for 0.2 and 0.5 mol% Ta2O5 loaded glasses and (b) DSC traces for 2–8 mol% Ta2O5 loaded glasses.
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with Bragg–Brentano flat plane geometry. The 2h scans were made between 5° and 60°, step width of 0.02° and a time per step of 4 s, with Cu Ka1 radiation
˚ ). Preparation of the samples used a sim(k = 1.54056 A ple top pack loading method for an acquired smooth surface.
Table 1 Thermal properties of the LZS glass containing Ta2O5 additions TMO
Tg(s) (°C)
Tg(mp) (°C)
Tx1(s) (°C)
DTTS (K)
Tx1 (°C)
Tx2 (°C)
Tliq (°C)
Tg(s)/Tliq (K/K)
None Ta(0.2) Ta(0.5) Ta(1) Ta(1.5) Ta(2) Ta(3) Ta(4) Ta(5) Ta(6.5) Ta(8)
442 ± 1 444 ± 2 447 ± 2 447 ± 3 452 ± 2 457 ± 1 459 ± 1 468 ± 3 475 ± 2 488 ± 5 498 ± 2
452 ± 1 455 ± 1 457 ± 1 458 ± 0 464 ± 1 467 ± 0 471 ± 0 481 ± 1 491 ± 1 505 ± 0 515 ± 2
621 ± 1 626 ± 1 636 ± 1 642 ± 1 666 ± 1 678 ± 1 691 ± 1 729 ± 3 722 ± 9 Not resolved Not resolved
169 171 179 184 202 211 220 248 231 – –
655 ± 1 660 ± 2 670 ± 1 678 ± 0 700 ± 1 713 ± 1 729 ± 1 772 ± 2 797 ± 5 783 ± 6 821 ± 26
720 ± 1 732 ± 0 732 ± 0 738 ± 1 756 ± 1 772 ± 2 No peak No peak No peak No peak No peak
951 ± 1 944 ± 1 944 ± 1 940 ± 0 940 ± 1 938 ± 0 938 ± 1 932 ± 1 933 ± 2 933 ± 7 985 ± 15
0.584 0.598 0.600 0.603 0.608 0.611 0.614 0.626 0.633 0.645 0.626
Tg(s) is the glass transition start temperature; Tg(mp) is the corresponding mid-point temperature; Tx(s) is the extrapolated peak start of the first or only crystallization exotherm; Tx1,2 are the peak crystallization temperatures; Tliq is the end of the melting range. DTTS is the working range defined as Tx1(s)– Tg(mp), and Tg(mp)/Tliq is the reduced glass temperature. Quoted uncertainties are the standard deviations.
Fig. 3. DSC traces for as-quenched and nucleated LZS glass containing Ta2O5. (a) DSC traces for as-quenched and nucleated LZS glass containing 0.5 mol% Ta2O5. Glass nucleated for 1 h at 465 °C. (b) Traces for as-quenched and nucleated LZS glass containing 3 mol% Ta2O5. Glass nucleated for 1 h at 475 °C.
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3. Results 30
peak 1
20 10
peak 2
0 420
440
460
480
500
520
Nucleation temperature (°C) 60
Tnuc (K)
50 peak 1
40 30 20
peak 2
10 0 420
440
460
480
500
520
Nucleation temperature (°C) 60 50 peak 1
Tnuc (K)
40 30 20 No second peak
10 0 420
440
460
480
500
520
Nucleation temperature (°C) Fig. 4. Plots of variation in peak crystallization temperature between asquenched and nucleated LZS glasses as a function of the nucleating temperature. (a) Plot of variation in peak crystallization temperature between as-quenched and nucleated unmodified LZS glass as a function of the nucleating temperature. (b) Plot of variation in peak crystallization temperature between as-quenched and nucleated LZS glass doped with 2 mol% Ta2O5 as a function of the nucleating temperature. (c) Plot of variation in peak crystallization temperature between as-quenched and nucleated LZS glass doped with 3 mol% Ta2O5 as a function of the nucleating temperature.
500 490
Tnuc (K)
All the glass compositions examined were noted to be fully amorphous as determined by XRD, even at the 8 mol% Ta2O5 level. Representative patterns are shown in Fig. 1. A shoulder can be observed on the high angle side of the amorphous peak as the concentration of Ta2O5 increases. This may indicate a degree of structural ordering in these higher Ta2O5 glasses. Representative DSC traces of a selection of the glasses are shown in Fig. 2 and the thermal properties are summarized in Table 1. The strong influence of Ta additions on the glass transition temperature of the LZS glass is also summarized in Table 1, Tg(s) increasing from around 442 °C to 498 °C for an 8 mol% addition. The DSC traces shown in Fig. 2 illustrate the increasing stabilizing influence of Ta additions on crystallization, where it may be noted that crystallization is virtually eliminated for concentrations >5 mol%. The heat of crystallization, DH, of the first crystallization peak for the LZS glass is 213 ± 3 J/g and for the second 40 ± 2 J/g, whilst a value of around 4 J/g is noted for the small single peak observed with the 8% Ta sample. As summarized in Table 1 the working range of the glasses as measured by DTTS is improved by the Ta additions. The GFA of the glasses, as defined by the reduced glass temperature, is marginally enhanced by all additions. In general, the nucleating efficiency of the as-quenched glasses is very good, with DT values between coarse and fine glass samples generally <10 K, this suggesting predominantly bulk crystallization. The influence of specific nucleation treatments on the glasses is nevertheless also very marked, as illustrated by the DSC traces shown in Fig. 3, and in Fig. 4 which shows Marotta plots of DTnuc against the temperature of nucleation, Tnuc. Very clear maxima are noted in these plots from which the optimum nucleation temperature for each composition can be derived. A substantial increase in optimum nucleation temperature with Ta2O5 loading is observed from around 465 °C to 490 °C, as shown in Fig. 5. Activation energies for crystallization are given in Table 2 and representative activation energy plots are shown in Fig. 6. The crystallization activation energies for asquenched glasses are reduced by all additions. It is particularly noteworthy that addition of Ta appears to suppress the formation of the high expansion silica phases. The activation energy for structural relaxation around the glass transition for the unmodified LZS glass was determined as 568 ± 15 kJ/mol whilst the value for samples loaded with 5% Ta2O5 was found to be significantly higher at 654 ± 25 kJ/mol. Structural relaxation activation energy plots comparing LZS glass with glass containing 5 mol% Ta2O5 addition are shown in Fig. 7. Summarized in Table 3 are the crystalline phases that form in samples heat-treated under standard conditions of 5 min at 950 °C to simulate a sealing cycle, followed by nucleation for 60 min at 465 °C and crystallization for 60 min at either 700 °C or 800 °C. The major crystalline
Tnuc (K)
40
480 470 460 450
0
1
2
3
4
5
6
Ta2O5 (mol%) Fig. 5. Optimum nucleation temperature as a function of Ta2O5 doping.
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Table 2 Crystallization activation energies for glasses containing TMO additions TMO
E1 (kJ/mol)
E2 (kJ/mol)
None Ta(0.2) Ta(0.5) Ta(1) Ta(1.5) Ta(2) Ta(4) Ta(5)
237 ± 2 159 ± 7 152 ± 5 157 ± 4 160 ± 3 158 ± 5 155 ± 4 200 ± 13
304 ± 2 275 ± 5 286 ± 12 276 ± 19 240 ± 12 164 ± 17a No peak No peak
a Only resolved at low heating rates 65 K/min. Quoted uncertainties are the standard deviations.
-10.5
5 4
2 1.5
1 0.5 0.2 mol% Ta2O5
ln (β/Tp2)
-11
phases include the silica polymorphs tridymite (PDF No. 83-1339) and cristobalite (PDF No. 39-1425), together with a Li2ZnSiO4 phase (PDF No. 15-0056), most probably a solid solution, and some residual glassy phases, as depicted by the XRD patterns shown in Fig. 8. There is considerably more tridymite present in the unmodified LZS crystallized at 800 °C. At the higher concentrations of Ta2O5, P2 mol%, tridymite is absent in samples crystallized at either 700 °C or 800 °C, and the proportion of cristobalite is reduced; whilst at higher concentrations still, the Li2ZnSiO4 phase is also substantially reduced and LiTaO3 (PDF No. 29-0830) and LiTaSiO5 (PDF No. 45-0644) phases appear. For 8 mol% Ta2O5 addition, samples heat treated at 700 °C are still XRD amorphous. Representative micrographs are shown in Fig. 9, which illustrate the pronounced effect Ta2O5 has on the crystallization behavior, including phase distribution and morphology. 4. Discussion
-11.5 -12 -12.5 -13 -13.5 0.9
0.95
1
1.05
1.1
1.15
1 / T (10-3K-1) Fig. 6. Representative activation energy plots for glasses containing Ta2O5 in the range 0.2–5 mol%.
3.5
ln(β)
LZS
5mol% Ta2O5
3 2.5 2 1.5 1.28
1.3
1.32
1.34
1.36
1.38
1.4
1 / Tg (10-3K-1) Fig. 7. Structural relaxation activation energy plots for LZS glass and glass doped with 5 mol% Ta2O5.
There have been very few reports detailing the effects of TMO additions, and in particular Ta2O5, on the properties of oxide glasses, and these have usually concentrated on a very small number of individual metals. For example, in the case of silicate glasses, the influence of Ti, Co, Fe, Mn, Ni and Co on the thermal conductivity of a barium borosilicate glass, where it was shown that the conductivity increases with increasing atomic weight of the metallic species [22]; the addition of Cr to a lithium silicate composition in order to increase solar absorbance [23]; and the effect of various TMO additions at the 2 mol% level on the properties of a LZS glass [11]. Some studies have also been reported for non-silicate glasses. These include phosphates where a limited number of TMO additions have been noted to influence the structure of the glasses [24– 26]; borate glasses where the addition of Fe, Co and Ni was noted to influence the electrical conductivity [27]; vanadate glasses where TMO additions were noted to enhance diffusion [28]; and tellurite glasses where TMO additions influenced the structure [29]. Differential scanning calorimeter traces of the lithium zinc silicate glass employed in the current investigation exhibit two very well-defined exotherms. It is known from previous studies [11] that the first exotherm corresponds to
Table 3 Crystalline phases formed in heat-treated glasses TMO
Crystalline phases 700 °C
Crystalline phases 800 °C
None Ta(0.2) Ta(0.5) Ta(1.0) Ta(1.5) Ta(2.0) Ta(4.0) Ta(6.5) Ta(8.0)
Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + Li2ZnSiO4 + LiTaO3 Amorphous
Tridymite + Cristobalite + Li2ZnSiO4 Tridymite + Cristobalite + Li2ZnSiO4 Tridymite + Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + Li2ZnSiO4 + LiTaO3 Cristobalite + LiTaSiO5 + LiTaO3
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307
Lithium Zinc Silicate, Li2 ZnSiO4 Cristobalite, SiO2 Tridymite, SiO2
Intensity
800°C
700°C
15
20
30
Two Theta Lithium Zinc Silicate, Li2 ZnSiO4 Cristobalite, SiO2 Tridymite, SiO
Intensity
800°C
700°C
15
20
30
Two Theta Fig. 8. Representative XRD patterns of crystallized glasses (slight shifts from their file values are observed for some of the peaks and are most likely the result of solid solution influences). (a) Unmodified LZS glass crystallized at 700 °C and 800 °C. Note the absence of tridymite peaks at 700 °C but the strong peaks at 800 °C. (b) LZS glass with addition of 2 mol% Ta2O5 crystallized at 700 °C and 800 °C. Note that tridymite is absent at both temperatures. (c) All LZS glasses from 0 to 8 mol% Ta2O5 crystallized at 700 °C. Top pattern is for the unmodified LZS; this is followed by 0.2, 0.5, 1, 2, 4, 6.5 and 8 mol% Ta2O5 modified glasses. Note the absence of tridymite peaks and the decreasing peaks from LZS and cristobalite with increasing Ta2O5, and the small proportion of LiTaO3 at the higher concentrations (4% and 6.5%); also note that the 8% composition is still X-ray amorphous at this crystallization temperature. (d) All LZS glasses crystallized at 800 °C. Top pattern is for the unmodified LZS; this is followed by 0.2, 0.5, 1, 2, 4, 6.5 and 8 mol% Ta2O5 modified glasses. Note the decreasing peaks from tridymite, cristobalite and LZS and with increasing Ta2O5 content; and also note the increasing proportions of LiTaSiO5 and LiTaO3 crystalline phases.
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Lithium Zinc Silicate, Li2ZnSiO4 Cristobalite, SiO2
Intensity
Lithium Tantalum Oxide, LiTaO3
LZS 0.2% 0.5% 1.0% 2.0% 4.0% 6.5% 8.0 %
20
25
30
35
40
Two Theta
Lithium Zinc Silicate, Li2SiO4 Cristobalite, SiO2 Tridymite, SiO2 Lithium Tantalum Oxide, LiTaO3 Lithium Tantalum Silicate, LiTaSiO5
Intensity
LZS 0.2% 0.5% 1.0% 2.0% 4.0% 6.5% 8.0 %
20
25
30
35
40
Two Theta Fig. 8 (continued)
the formation of a Li2ZnSiO4 phase, whilst the second higher temperature exotherm yields silica phases, i.e., cristobalite, quartz or tridymite, or a combination of these. It is clear that the addition of Ta2O5 to this glass can have a very pronounced influence on the thermal properties and crystallization behavior, as is evident from Tables 1–3 and Figs. 2–5, 8 and 9. Most noteworthy is the ability of Ta to suppress crystallization of the higher expansion silica phases, whilst at higher concentrations crystallization of Li2ZnSiO4 is also almost entirely eliminated and a small proportion of LiTaO3 and LiTaSiO5 phases are formed. The precise reason why Ta should have such a pronounced influence on the stability of the glass to crystallization is not clear at this time, but may be related to the ability of
this refractory TMO to enter into the structure in a network former or intermediate role, this being borne out by the marked increase in the glass transition temperature with Ta additions, noted in Table 1. Activation energies for crystallization are reduced by all Ta2O5 additions, as summarized in Table 2. It should be noted that the values shown in this table are the Kissinger values, generally applicable for surface crystallization, and therefore do not take into account the crystallization mechanism. This is considered valid for a qualitative comparison between the unmodified LZS and the various Ta2O5 additions, as the overall crystallization mechanism is likely to be similar for all these compositions. It is noteworthy that use of the Matusita and Saaka modified equation
I.W. Donald et al. / Journal of Non-Crystalline Solids 354 (2008) 301–310
using values for n = m = 3, i.e. bulk crystallization, yields values for apparent activation energy approximately only 3–5% higher than the unmodified Kissinger case, and this is normally well within the experimental error associated with such values [18]. Isothermal studies on this LZS glass in the as-quenched state [13] indicate that n corresponding to the first crystallization exotherm is in the range 1.6–2.1
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over the temperature range 635–660 °C. In the present study, using Eq. (2) a value for n in the range 1.7–2.4 over the same temperature range is determined, which is in very good agreement with the isothermal method. The major nucleating agent in this system is P2O5 and it is known [11] that, on nucleation, glass-in-glass phase separation occurs on a very fine nanometer scale. The precise effect of Ta2O5 additions on this behavior is not yet clear, but nucleation performance is undoubtedly enhanced at low concentrations, as is evident by the increase in the values for DT noted in the Marotta plots shown in Fig. 4, but is suppressed at higher concentrations, >5 mol%, when crystallization is very much less pronounced. The current work has indicated that the precise silica phases formed and their relative proportions are very dependent on the processing conditions, and particularly on the crystallization temperature employed and the amount of Ta2O5 present. As noted in Table 3 and illustrated by Fig. 9 which shows SEM micrographs of crystallized unmodified LZS and crystallized 3 and 4 mol% Ta2O5 samples, addition of Ta2O5 does indeed have a very pronounced influence on the crystallization behavior including the phase distribution and morphology. The findings of this work may have important implications for the development of glass-ceramics with tailored thermal expansion characteristics. By modifying the proportions of the intermediate and high expansion phases it may be possible to produce glass-ceramics in the lithium zinc silicate system, and in ostensibly the same LZS composition, with a wide variety of characteristics to match many different materials, both metals and ceramics. The influence of composition on thermal expansion behavior will be reported in a subsequent publication. 5. Conclusions
Fig. 9. SEM micrographs of glasses crystallized at 800 °C. (a) Unmodified LZS crystallized glass (phases present include lithium zinc silicate together with cristobalite/tridymite and residual glass). (b) Crystallized LZS glass with 3 mol% Ta2O5 addition (phases present include lithium zinc silicate, cristobalite/tridymite, lithium tantalum oxide and residual glass). (c) LZS with 4 mol% Ta2O5 addition (phases present as (b) together with lithium tantalum silicate).
(1) It is confirmed that the lithium zinc silicate glass exhibits two crystallization exotherms which correspond to the formation of lithium zinc silicate and silica phases, respectively. (2) Addition of Ta2O5 suppresses the formation of high thermal expansion silica phases, whilst at higher concentrations, it also inhibits crystallization of the Li2ZnSiO4 phase. These phases are replaced by small amounts of LiTaO3 and LiTaSiO4. (3) Thermal properties, including the glass transition temperature, glass-forming ability, crystallization temperature and optimum nucleation temperature corresponding to the maximum nucleation rate, are all increased by addition of Ta2O5. (4) Nucleation of glasses at temperatures around the glass transition leads to very significant reductions in the crystallization temperature, as noted by DSC. (5) Activation energies for crystallization are significantly reduced by all Ta2O5 additions, whilst activation energy for structural relaxation is increased.
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